A diode used in power converters must have a low on-state voltage and high switching speed. At a low voltage application, a Schottky diode is generally used because it offers these characteristics. However, if a Schottky diode is designed for a high voltage application, the on-voltage increases a lot, and the loss resulting from the leakage current increases. Therefore, a PIN-type diode is usually used for high voltage application. This PIN-type diode has a so-called conductivity-modulating layer, and can be used under a high voltage, thereby reducing the on-voltage. However, a large reverse recovery current flows with a reverse recovery because of excessive carriers in the conductivity-modulating layer. FIG. 8 shows the changes that take place in the current during reverse recovery in the PIN-type diode. When a current is decreased at a time t1 with certain di/dt, the voltage decreases as the current decreases. When a reverse voltage is applied, the carriers injected into the conductivity-modulating layer are swept out, and a large reverse recovery current begins to flow. Therefore, when high-speed switching is performed by using the PIN-type diode, a problem arises in that the switching loss increases.
To reduce the reverse recovery current in the PIN-type diode, a lifetime killer (i.e., a recombination center) may be introduced to quickly recombine the excessive carriers during reverse recovery. However, introducing a lifetime killer to accelerate carrier recombination creates a problem in that it reduces the carriers and raises the on-state voltage as a result of the recombination even if the PIN-type diode is in an on-state. Thus, in the PIN-type diode, the on-state voltage reduction is in a trade-off relation with the switching loss.
To solve this problem, a proposal has been made for a diode which suppresses this increase in leakage current, a problem that exists in the Schottky diode, which is constructed as shown in FIG. 9. This diode has an n.sup.- -type semiconductor layer 2 formed on an n.sup.+ -type semiconductor substrate 1, on which a p.sup.+ -type anode layers 3 are formed in a dispersed manner. Further-more, a cathode electrode 9 is connected to the n.sup.+ -type semiconductor substrate 1, and an anode electrode 8 is connected across from the n.sup.- -type semiconductor layer 2 to the p.sup.+ -type anode layers 3. This anode electrode 8 is selected so that it makes a Schottky junction with the n.sup.- -type semiconductor layer 2 and an ohmic junction with the p.sup.+ -type anode layers 3. Therefore, when a voltage in the reverse direction is applied to the anode electrode 8 and the cathode electrode 9, the anode electrode 8 is isolated electrically from the cathode electrode 9 as a result of the Schottky junction and the p-n junction. The space between each anode layer 3 is determined in a way that the depletion layers 11 spreading from the anode layers 3 are pinched off the surface of the n.sup.- -type semiconductor layer 2. Therefore, the region in which the anode electrode 8 is connected with the n.sup.- -type semiconductor layer 2 has the depletion layers 11 spread, whereas a leakage current from the Schottky junction under a state in which a reverse recovery voltage is applied is suppressed. Thus, this diode is a diode that offers high-speed performance of the Schottky diode and a lower leakage current. However, because the n.sup.- -type semiconductor layer 2 does not enter a conductivity-modulating condition, this type of device with high withstand voltage has high on-state voltage.
In order to reduce the on-state voltage in such a diode, a diode, which has a structure as shown in FIG. 10, is proposed. This diode has the same basic structure as the diode shown in FIG. 9, except the width of the anode layers 3 is increased. Such an element is commonly called a spin diode, and has a high-speed performance of the Schottky diode in a region with less current, developing the conductivity modulation in a region with a larger current, and its behavior resembles a pin-type diode. That is, when a forward voltage is applied and a large current flows into the cathode electrode 9 from the junction of the n.sup.- -type semiconductor layer 2, a voltage drop is generated because the electron current components flow transversely directly below the p.sup.+ -type anode layers 3. In this diode, since the width of the anode layer 3 is great, the junction of the p.sup.+ -type anode layers 3 and the n.sup.- -type semiconductor layer 2 is biased in the forward direction. Therefore, holes are injected from the p.sup.+ -type anode layers 3 into the n.sup.- -type semiconductor layer 2 to develop a conductivity modulation.
Thus, in the diode shown in FIG. 10, the on-state voltage can be reduced because the n.sup.- -type semiconductor layer 2 performs the conductivity modulation. However, its on-voltage reducing effect is only slight because its injection efficiency is not as high and the conductivity modulation is not as great as it is in the pin-type diode. Nevertheless, a conductivity modulation, which is not as large, reduces the excessive carrier amount, and can recover at a relatively high speed. Therefore, characteristically it leads to behavior that is intermediate between a Schottky diode and a pin-type diode. As the anode layer 3 width is increased, the diode shows characteristics that more closely resemble those of the pin-type diode. However, it is not possible to realize diodes that have superior characteristics possessed by Schottky diodes and pin-type diodes, that is, combining a low on-state voltage and high-speed performance.
It is of course possible to achieve a higher speed by introducing a lifetime killer into these diodes, but since the trade-off relation between the on-state voltage and switching loss is for an improvement, no low on-state voltage and low switching loss can be realized.
Accordingly, in the light of the above problems, the present invention is intended to realize a diode element that can simultaneously realize a low on-state voltage and a low switching loss.